The present invention relates to a magnetic record reproducing apparatus in which digital signal processing is produced during magnetic recording or reproduction of a video signal, and in particular to a reproducing system for a chrominance signal in a digital video tape recorder.
FIG. 3 shows a magnetic record reproducing apparatus 101 in which the video signal processing required to reproduce a magnetic record is performed on digitized signals. A composite video signal Y+C is composed of luminance signal Y and chrominance signal C, and is provided to A/D converter 103 through change-over switch 102. The position of switch 102 depends on whether the device is in record (R) or reproducing (play or P) mode. The composite video signal is converted to a digital signal by A/D converter 103 and is supplied to digital signal processing circuit 104. Digital signal processing circuit 104 processes the signal to modulate a frequency of luminance signal Y. The video signal processed by digital signal processing circuit 104 is converted to an analog signal by D/A converter 105 and thereafter is supplied to rotary magnetic head 107 through changeover switch 106, switched depending on whether the device is in the record or play mode, and is then recorded on or reproduced from magnetic tape 108.
The video signal read out from magnetic tape 108 by rotary magnetic head 107 is supplied to digital signal processing circuit 104 through changeover switch 102 and A/D converter 103, thereby performing a signal processing which is the reverse of the processing at the recording time with respect to the video signal. The composite video signal thus digitized is converted to an analog signal by D/A converter 105 and thereafter is supplied to an external monitor 199 to display the image received through changeover switch 106.
In the above-mentioned conventional magnetic record reproducing apparatus 101, luminance signal Y and chrominance signal C are A/D-converted, as they are composite video signals. Accordingly, it is necessary to dispose a luminance chrominance separating circuit, an automatic gain control (AGC) circuit or an automatic color gain control (ACC) circuit, etc., within digital signal processing circuit 104. Therefore, this digital signal processing circuit 104 has included a large amount of circuitry to effect these functions, and therefore has tended to be large in size. Further, when a sampling frequency suitable for the signal processing of both the luminance signal and the chrominance signal is selected, the band from which the sampling frequency in A/D converter 103 can be selected becomes narrowed. Further, an output of a tuner circuit for selecting a television signal to be magnetically recorded is varied by this selection of a proper sampling frequency, and it is sometimes difficult to maintain the quality of the chrominance signal using the automatic color gain control circuit when the voltage level of the chrominance signal is reduced. Further, when a clock signal required to perform the sampling operation of the video signal is set to have a frequency which is an integer times a color subcarrier frequency but is not phase-locked with a horizontal synch signal, it is not possible to perform noise reduction of a cyclic type utilizing line correlation and frame correlation, or to perform special image processing for providing a special effect such as a strobe effect, a mosaic effect, a solarization effect, or the like.
To solve the problems discussed above, the present invention proposes a magnetic record reproducing apparatus comprising a pair of A/D converters for respectively digitizing luminance and chrominance signals as components of an analog composite video signal, a digital signal processing circuit for receiving the luminance and chrominance signals digitized by the pair of A/D converters and processing the signals for modulating a frequency of the luminance signal and for converting a frequency of the chrominance signal into a low frequency band, and a D/A converter for receiving the luminance signal modulated with respect to the frequency and the chrominance signal converted in the low frequency band, said luminance and chrominance signals being mixed with each other within the digital signal processing circuit, said D/A converter converting both signals to analog signals to perform a magnetic recording operation.
In the present invention, the luminance and chrominance signals are separately digitized and the chrominance signal is modulated with respect to frequency and the frequency of the chrominance signal is converted in the low frequency band within the digital signal processing circuit. Thereafter, these signals are mixed with each other, and the mixed signal is converted to an analog signal by the common D/A converter to perform the magnetic record operation. Accordingly, the operating burden of the digital signal processing circuit is reduced, the selecting band of a sampling frequency is secured, and the accuracy in D/A conversion by the luminance and chrominance signals is secured.
Another aspect of the invention relates to video tape recorders of the consumer type which have recently come into wide use, and especially those that make extensive use of digital technology. One such is disclosed in an article entitled "Digital Signal Processing in Video Tape Recorders" IEEE Trans. on Consumer Electronics Vol. CE-31 August 1985, for example.
This article describes system clock signals for the respective digital circuits being formed by an independent oscillating circuit irrespective of any correlation with respect to a horizontal synch signal of an input video signal or a color subcarrier signal. The frequency of the system clock signal is selected independent of a horizontal synchronization frequency and a color subcarrier frequency.
However, in such a video tape recorder which uses a system clock signal with a frequency having no correlation with the input video signal, it is difficult to perform signal processing with proper correlation to the video signal (television signal) for the following reasons.
(a) It is possible to reduce noise using a frame correlation. However, if this is done, it is necessary that the positions of a picture element on horizontally and vertically two dimensional planes of the video signal are equal to each other before and after one frame, but are not aligned with each other when the system clock signal having no correlation with the video signal is used.
(b) It is well known that, when a chrominance signal component of the video signal as in NTSC, PAL or SECAM systems, etc. is converted to a primary chrominance signal, the circuitry can be made smaller-sized by performing a sampling operation by the system clock signal having a frequency four times the color subcarrier frequency phase-locked with respect to a color burst signal. However, such a method cannot be used with a system clock signal having no correlation mentioned above.
It has also been to use a system clock signal phase-locked with the horizontal synch signal and the color subcarrier signal and having a frequency an integer times the horizontal synch frequency or the color subcarrier frequency as in the case in which the system clock signal is used in a digital television image receiver and a video tape recorder for business.
However, the frequency of such a system clock signal is high and the respective circuits are complicated and large in size, so that it is not sufficient to just apply the system clock signal to a known video tape recorder.
The second aspect of the present invention has an object of providing a consumer--type video tape recorder simplified in construction as much as possible when the recorder is converted to digital technology.
To solve the problems mentioned above, the present invention defines a digital video tape recorder for picking up and reproducing, from a magnetic tape, a video signal in which a chrominance signal is converted to a low frequency band, said tape recorder being characterized in that a clock signal synchronized with a color burst signal of said picked-up video signal and having a frequency 4.times.N (where N is a natural number) times a carrier frequency converted into the low frequency band is applied as a clock signal with respect to a circuit constituting at least a portion of a chrominance signal reproducing system.
The clock signal with respect to a digital circuit (including an analog/digital converting circuit) for processing the chrominance signal converted into the low frequency band is synchronized with the color burst signal of the picked-up video signal and has the frequency 4.times.N (where N is a natural number) the carrier frequency converted into the low frequency band. Accordingly, it becomes possible to process the video signal while maintaining a correlation with the clock signal, and a color difference signal component can be determined even when the chrominance signal is converted into the low frequency band, thereby simplifying the signal processing.